Recently, an internal combustion engine is equipped with a turbocharger and an intercooler in order to reduce fuel consumption and obtain a large output. In such an internal combustion engine, a compressor of the turbocharger draws in and compresses exhaust gas or outside air, so that supercharged air generated during this process is supplied to the engine.
FIG. 5 shows an engine having a conventional turbocharger. Supercharged air passes through a turbocharger 20 and is supplied to a combustion chamber 11 inside of a cylinder 16 of an engine 10 through an intake port 12, and exhaust gas combusted in the combustion chamber 11 discharges through an exhaust port 13.
However, in the engine 10 having the turbocharger 20, an intake pressure in the intake port 12 is larger than an exhaust pressure in the exhaust port 13 in some portion by boost pressure rise in a low speed and high load region. In this case, fresh air 30 is supplied through the intake port 12 and directly transferred to the exhaust port 13 in the portion that an intake valve 14 and an exhaust valve 15 are overlapped, as shown in FIG. 5.
Since scavenging efficiency increases under a scavenging state so that the residual gas amount in the combustion chamber 11 reduces, fuel charging efficiency increases, and the torque increases.
The amount of fresh air 30 transferal to the exhaust port 13 increases, however, under the scavenging state, an oxygen sensor installed around a catalyst device 40 may recognize that combustion is in a lean state. Due to this determination result, a fuel amount being supplied to the combustion chamber 11 increases so that the amount of un-burn fuel may increase in the combustion chamber 11. If the un-burn fuel is directly transferred to a catalyst through the exhaust port 13, the un-burn fuel reacts with the fresh air so that after-combustion may be occurred in the catalyst device 40. Accordingly, the temperature of the catalyst increases so that the catalyst device 40 may be damaged.
The foregoing is intended merely to aid in the understanding of the background of the present disclosure, and is not intended to mean that the present disclosure falls within the purview of the related art that is already known to those skilled in the art.